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composition of that mass of each of these which is produced by combining 10.3 parts of phosphorus with sulphur is stated, we have

(1) 8 x 2(=16) parts by weight of 10.3 parts by weight of phos- sulphur. phorus combine with (2) 8x3} (=26:6)

sulphur. Chlorine and oxygen form two compounds, the compositions

of which are;

(1) 8 parts by weight of 35.5 parts by weight of chlorine oxygen. combine with

(2) 8x4 (=32) parts by weight

of oxygen. Chlorine and sulphur form a compound the composition of which is; 35:5 parts by weight of chlorine combine with 8x4 (=32) parts

by weight of sulphur. These results may be stated in more general terms thus:-

The masses of phosphorus, oxygen, sulphur, and chlorine, which severally combine with a constant mass of hydrogen are also the masses of those elements which combine with each other, or they bear a simple relation to these masses.

This statement, or a statement equivalent to this, holds good 71 for all the elements. The statement is known as The law of reciprocal proportions.

This law may be expressed in various forms; thus

When two elements, A and B, severally combine with a third element, C, then the proportions in which masses of A and B severally combine with C are also the proportions in which A and B combine with each other, or they bear a simple relation to these proportions. Or, better, thus

The masses of different elements which severally combine with one and the same mass of another element are also the masses of those different elements which combine with each other, or they bear a simple relation to these masses.

The student should particularly observe that the laws of 72 multiple, and reciprocal, proportions, are generalised statements of facts. He should also familiarise himself with the method by which these laws are deduced from the composition of compounds. Statements of the percentage composition of a series of compounds do not suggest the laws in question, although

73

they contain the data from which the laws are deduced. It is necessary to compare the compositions of compounds of each of two, or more, elements with one and the same element; it is also necessary to state these compositions so that the mass of the element with which the others combine is kept constant throughout all the compounds.

Any element may be chosen as the standard element; and any mass of the standard element may be chosen as the fixed mass with which other elements are to be combined. It is found that the relations between the masses of elements which mutually combine are very clearly and simply exhibited by choosing hydrogen as the standard element, and one part by weight (say 1 gram) of hydrogen as the fixed mass.

The following table illustrates this way of looking at the composition of several compounds. Column I. exhibits the composition of three compounds of hydrogen, stated, (a) as parts of each element per 100 parts of the compound, (6) so as to shew the weight of the second element combined with 1 part by weight of hydrogen. Columns II., III., and IV., exhibit the composition of compounds of two elements neither of which is hydrogen, stated, (a) as parts per 100, and (b) so as to shew the weights of those elements which severally combine with that weight of oxygen, sulphur, or chlorine, which has been shewn in I. to unite with 1 part by weight of hydrogen.

The masses of oxygen, sulphur, and chlorine, which severally combine with 1 part by weight-i.e. with unit mass -of hydrogen are 8, 16, and 35.5, respectively. The masses of copper, lead, and thallium, which severally combine with 8 parts by weight of oxygen are 31.7, 103.5, and 204, respectively; and these are also the masses of those elements which severally combine with 16 parts by weight of sulphur, and with 35.5 parts by weight of chlorine, respectively.

Let us call those masses of oxygen, sulphur, &c. the combining weights of oxygen, sulphur, &c. We have then :

Combining weights, deduced from composition of compounds with hydrogen.

Oxygen = 8; Sulphur = 16; Chlorine = 35.5. These numbers represent parts by weight of each element which combine with one part by weight of hydrogen.

Combining weights, deduced from composition of compounds with oxygen.

Copper = 31:7; Lead = 103.5; Thallium = 204.

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III. Copper Sulphide

Sulphur 33:53 (a) Copper

100.00 (6)

Sulphur = 16 Copper 31.7

=

Lead Oxide

Oxygen (a)

Lead

Lead Sulphide (a)

Sulphur = 13:39 Lead

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Hydrogen Sulphide

Hydrogen = 5.88 (a) Sulphur 94:12

100.00 Hydrogen

1 (6)

Sulphur 16

100.00

8 = 103.5

100.00 Sulphur = 16 Lead = 103.5

Oxygen Lead

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(6)

(6)

Hydrogen Chloride

Hydrogen = 2.74 (a) Chlorine

= 97.26 100.00

1 Hydrogen = 6

35.5

Thallium Oxide

Oxygen 3.77 (a) Thallium 96.23

100.00 Oxygen

8 (6)

Thallium = 204

66.47

IV.
Copper Chloride
Chlorine 52.86
Copper

47.14

100.00
Chlorine 35.5
Copper

31.7

86.61

Lead Chloride

Chlorine 25:54
Lead 74.46

100.00
Chlorine 35.5
Lead = 103.5

Thallium Sulphide
Sulphur

7.27
Thallium = 92.73

100.00
Sulphur 16
Thallium = 204

Thallium Chloride
Chlorine 14.82
Thallium 85.18

100.00
Chlorine 35.5
Thallium = 204

(b) Chlorine

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they contain the data from which the laws are deduced. It is necessary to compare the compositions of compounds of each of two, or more, elements with one and the same element; it is also necessary to state these compositions so that the mass of the element with which the others combine is kept constant throughout all the compounds.

Any element may be chosen as the standard element; and any mass of the standard element may be chosen as the fixed mass with which other elements are to be combined. It is found that the relations between the masses of elements which mutually combine are very clearly and simply exhibited by choosing hydrogen as the standard element, and one part by

weight (say 1 gram) of hydrogen as the fixed mass. 73 The following table illustrates this way of looking at the

composition of several compounds. Column I. exhibits the composition of three compounds of hydrogen, stated, (a) as parts of each element per 100 parts of the compound, (6) so as to shew the weight of the second element combined with 1 part by weight of hydrogen. Columns II., III., and IV., exhibit the composition of compounds of two elements neither of which is hydrogen, stated, (a) as parts per 100, and (b) so as to shew the weights of those elements which severally combine with that weight of oxygen, sulphur, or chlorine, which has

been shewn in I. to unite with 1 part by weight of hydrogen. 74

The masses of oxygen, sulphur, and chlorine, which severally combine with 1 part by weight-i.e. with unit mass —of hydrogen are 8, 16, and 35.5, respectively. The masses of copper, lead, and thallium, which severally combine with 8 parts by weight of oxygen are 31:7, 103.5, and 204, respectively; and these are also the masses of those elements which severally combine with 16 parts by weight of sulphur, and with 35.5 parts by weight of chlorine, respectively. Let us call those masses of oxygen, sulnha

the combining weights of oxygen, sulphur. f

Combining weights compounds with hy

Oxyge These nu · which combi - Combi compoun

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(a) Lead

7:18 92.82

(a) Lead

Lead Sulphide Sulphur = 13:39

100.00 (6)

Hydrogen Sulphide

Hydrogen = 5.88 (a) Sulphur

94:12

100.00 Hydrogen = 1 (6)

Sulphur 16

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